This invention relates generally to heat shrinkable, relatively gas impermeable, thermoplastic packaging film which can be heat sealed to itself to form a flexible package. The invention relates more particularly to bags of such film for food products in which the packaged product is submerged in near boiling water or autoclaved for a substantial period of time for pasteurizing or cooking, the bag structure being shrinkable and nondegradable under such conditions.
There is a need in the food packaging industry for a packaging film from which bags can be made which are of improved structural soundness such that they may be fully characterized as pasteurizable and/or cook-in. It is desirable to have a precooked food product after the cooking process which is attractively packaged and maintained inside the film within which it was cooked.
The term "pasteurizable" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to pasteurizing conditions while containing a food product. Many food products require pasteurization after they have been hermetically packaged to destroy harmful microbes which grow in the absence of air. Specific pasteurization requirements tend to vary by country; however, representative conditions are submersion of the hermetically sealed product in water at 93.degree. C. for 3 minutes or at 85.degree. C. for 10 minutes or at 80.degree. C. for 20 minutes. Submersion at 95.degree. C. for 1 hour probably represents the limiting case. Thus, for a bag to be characterized as pasteurizable, structural integrity of the bag must be maintained during pasteurization, i.e. the bag must have superior high temperature seal strength and must be delamination resistant under such time-temperature conditions. Additionally, the packaging material should be heat shrinkable under pasteurizing conditions so as to provide an attractively packaged pasteurized food product.
The term "cook-in" as used herein is intended to refer to packaging material structurally capable of withstanding exposure to cook-in time-temperature conditions while containing a food product. Cook-in packaged foods are essentially pre-packaged, pre-cooked foods that go directly to the consumer in that configuration which may be consumed with or without warning. Cook-in time-temperature conditions typically refer to a long slow cook, for example submersion in water at 70.degree.-80.degree. C. for 4-6 hours. Such cook-in time-temperature requirements are representative of institutional cooking requirements. Submersion at 80.degree. C. for 12 hours probably represents the limiting case. Under such conditions, a packaging material properly characterized as cook-in will maintain seal integrity and will be delamination resistant. Additionally, the packaging film should be heat shrinkable under these conditions so as to form a tightly fitting package, and also should have good product adhesion to prevent "cook-out" or collection of juices between the surface of the food product and the interior surface of the packaging material.
Generalizing, there are a number of requirements for a pasteurizable, cook-in packaging material. It is the purpose of the present invention to provide a pasteurizable, cook-in packaging film meeting all of these requirements. First, bags made from such film must have seal integrity under such conditions, i.e. the heat sealed seams should resist being pulled apart during heat shrinking. As a corollary, the film should be heat sealable to itself. Second, such bags must be delamination resistant, i.e. the multilayers making up the film must not separate or blister. Third, the food contact layer of such film must qualify under the appropriate food laws and regulations for safe food contact. Fourth, the film must provide an oxygen and vapor barrier, i.e. must possess a low permeability to maintain the freshness of the food contained therein. Fifth, the film must be heat shrinkable in hot water under these time-temperature conditions, i.e. the film must possess sufficient shrink energy such that upon the packaged food product being submerged in hot water the packaging film will shrink snugly around the product contained therein, representatively about 30-50% biaxial shrinkage at about 90.degree. C. Sixth, the film should possess optical clarity, i.e. the film should not become cloudly upon exposure to these time-temperature conditions so as to maintain eye appeal of the packaged product.
In general, such a multilayer film structure will have the minimal structure (sealing and food contact layer)/(shrink layer)/(barrier layer)/(sealing and food contact layer), a composite structure being required to achieve the desired composite properties of the packaging film. A typical barrier material is saran which refers to a family of thermoplastic resins, viz vinylidene chloride polymers, and copolymers of vinylidene chloride with other monomers such as vinyl chloride, acrylonitrile, methyl metacrylate, acrylates, acrylate esters, vinyl acetate, etc. Copolymers of vinylidene chloride with other monomers are generally described in terms of the weight ratio of the monomer unitls in the copolymer. Saran, by definition, has at least 50% vinylidene chloride.
A heat shrinkable, thermoplastic, multilayer packaging film which has enjoyed considerable commercial success for making bags having low gas transmission is described in U.S. Pat. No. 3,741,253 issued on June 26, 1973 to Brax et al, which relates to a multilayer film comprising a first outside layer of an irradiated ethylene vinyl acetate copolymer, a core layer of polyvinylidene chloride copolymer, and a second outside layer of an ethylene vinyl acetate copolymer. Typically the process of manufacturing this type of oriented heat shrinkable film is a tubular orientation process wherein a primary tube of the film is biaxially oriented by stretching with internal pressure in the transverse direction and the use of pin rollers at different speeds in the machine direction. After the bubble is collapsed, the film is wound up as flattened, seamless, tubular film to be used later to make bags, e.g., either end-seal bags typically made by transversely heat sealing across the width of flattened tubing followed by severing the tubing so that the transverse seal forms the bottom of a bag, or side-seal bags in which the transverse seals from the bag sides and one edge of the tubing forms the bag bottom.
This type of bag is used by placing the food product in the bag, evacuating the bag, gathering and applying a metal clip around the gathered mouth of the bag to form a hermetic seal, and then immersing the bag in a hot water bath at approximately the same temperature at which the film was stretch-oriented, typically about 160.degree. to 205.degree. F., hot water immersion being one of the quickest and most economical means of transferring sufficient heat to the film to shrink it uniformly. One problem which has been encountered is the failure of the bag seals at the bottom of the bags as the bag is shrunk around a product, the shrink forces tending to pull the seal apart.
Of interest concerning the present invention, is the disclosure of U.S. Pat. No. 3,808,304 for "Oriented Blends of Polypropylene and Poly-Butene-1" issued Apr. 30, 1974 to Schirmer, being directed to packaging film made of such blends and having improved heat sealing and heat shrinking properties in near boiling water.
Of interest is the disclosure of U.S. Pat. No. 3,634,553 for "Heat Shrinkable Films of Polypropylene and an Ethylene/Butene Coploymer" issued Jan. 11, 1972 to Foglia et al, being directed to oriented thermoplastic films formed from blends of a high isotactic content polypropylene with an ethylene/butene-1 copolymer containing a minor amount of ethylene.
Of interest is the disclosure of U.S. Pat. No. 4,207,363 for "Flexible Heat Shrinkable Multilayer Film for Packaging Primal Meat" issued June 10, 1980 to Lustig et al, being directed to a heat shrinkable multilayer film including a first outer layer comprising a blend of a propylene-ethylene copolymer, a butene-1/ethylene copolymer, and a thermoplastic elastomer; a first core layer capable of being stretched during orientation of the multilayer film; a second core layer serving as a oxygen barrier and being compatable to the biaxial orienting and heat shrinking of the film; and a second outer layer comprising an ethylene/vinyl acetate copolymer; the multilayer film being biaxially oriented.
Of interest is the disclosure of U.S. Pat. No. 3,891,008 for "Multilayer Thermoplastic Film and Method for Making Same" issued June 24, 1975 to D'Entremont, being directed to a polymeric composition comprising a blend of polybutene-1 with an ethylene-propylene copolymer which, when combined with a shrinkable, cross-linked polymer of ethylene, produces a laminate having improved abuse and tear resistance and reduced self-adherence.
Of interest is the disclosure of U.S. Pat. No. 3,754,063 for "Method for Casting an Inflated Tubular Polyethylene Film with Mixed Polymer of Polypropylene-Polybutene-1" issued Aug. 21, 1973 to Schirmer, being directed to a thermoplastic film having improved abuse characteristics including a layer of ethylene vinyl acetate copolymer on a tubular substrate of a predominantly ethylene polymer material with an outer layer of admixed isotactic polypropylene, polybutene-1, and atactic polypropylene, the resulting film being oriented.